I noticed that Listbox loads an icon for each list element. This is not particulary scalable:) Usually we don't need to provide a distinct icon for a list item. So I think it makes sense for the icon object to be responsible for its own loading.
This new implementation of icons' behaviour is order of magnitude faster than the original one. For example a 1000 items list with 7 different types of icons loads in 30ms compared to 2s in the case of original implementation.
This diff is relative to 1.4.0 version. It was generated by examdiff application (I hope it's unix friendly:)